it's probably too low of a resistance. normally, car speakers are 4 ohm, so it would be 2 ohms of resistance per channel, which might be too low for your deck, causing the 'crackling.
it sounds like you wired them like the image on the right, i suggest you change it to be wired like the image on the left.
this will reduce the overall loudness of the system, but will work reliably. the only other solution is to get a deck with 4 outputs, and connect each speaker individually.